The phenomenon of sleep bruxism (SB) has been recognized and described for centuries, including literary references to the gnashing of teeth. Early etiologic explanations were generally focused on mechanistic factors, but later, attention was focused on psychologic issues such as stress and anxiety; by the end of the 20th century, most opinions combined these two ideas. However, recently, the study of the SB phenomena has occurred primarily in sleep laboratories in which patients could be observed and monitored over several nights. Various other physiologic systems were also studied in sleep laboratories, including brain activity, muscle activity, cardiac function, and breathing. As a result of these studies, most authorities now consider SB to be a primarily sleep-related movement disorder, and specific diagnostic criteria have been established for the formal diagnosis of that condition. All of these changes in the understanding of the SB phenomena have led to a corresponding change in thinking about how oral appliances (OAs) might be used in the management of SB. Originally, they were thought to be a temporary measure that could help dentists analyze improper dental relationships. Unfortunately, this often led to dental procedures to "improve" these relationships, including equilibrations, orthodontics, bite opening, or even major restorative dentistry. However, it is now understood that the proper role for OAs is to protect the teeth and hopefully to diminish muscle activity during sleep. This paper reviews these evolutionary changes in the understanding of SB and how this affects concepts of designing and using OAs.